Adaptive modulation is a technique for enhancing transmission efficiency and improving signal quality and the use efficiency of radio waves by adaptively varying the modulation scheme according to the radio reception state. Specifically, when the radio reception state is good, the error rate is kept low even in a modulation scheme in which the number of modulation multi-values (hereinafter referred to as the multi-value number) is high. Therefore, a modulation scheme having a high multi-value number is switched to, and throughput is maximized. If the radio reception state deteriorates, a reduction in communication quality (an increase in the error rate) is prevented by employing a modulation scheme having a low multi-value number.
FIG. 7 is a conceptual diagram showing a conventional wireless communication system that uses an adaptive modulation scheme. As shown in the diagram, the modulation scheme in the channel between the base station device and the terminal device is adaptively changed from the four-value modulation QPSK (Quadrature Phase Shift Keying) to the sixteen-value modulation 16QAM (Quadrature Amplitude Modulation), and then back to QPSK according to variation of the radio state. When the radio reception state deteriorates further, the modulation scheme of the channel is changed to the two-value modulation BPSK (Binary Phase Shift Keying), which is a modulation scheme having the smallest number of multi-values (hereinafter referred to as the minimum modulation scheme) in the system shown in the diagram. Although the throughput decreases when BPSK is utilized, errors are less likely to occur.
When the radio state further deteriorates, and the error rate correspondingly increases, it becomes impossible to maintain the required throughput even by BPSK, which is the minimum modulation scheme. In such a case, deterioration of communication quality is conventionally prevented by performing channel switching, handing over, or the like to another channel in which the radio reception state is good. Specifically, when the required communication quality becomes impossible to maintain merely by performing adaptive modulation in the same channel, the channel for communication is switched to another channel in which the reception state is good.
Patent Document 1 discloses a technique whereby wireless channel allocation is allowed only when the communication environment of a transmission channel is an environment that enables communication to be performed in accordance with a modulation scheme having a high multi-value number when there is a connection request from another wireless device in a wireless device that is adapted for adaptive modulation.
Patent Document 1: Japanese Laid-open Patent Application No. 2004-120051